1. Technical Field
The present invention relates to a line head and an image forming apparatus.
2. Related Art
Electrophotographic image forming apparatuses such as copying machines or printers are provided with an exposure unit that performs an exposure process on an outer surface of a rotating photoconductor so as to form an electrostatic latent image thereon. As the exposure unit, a line head having a structure in which a plurality of light-emitting elements is arranged in the direction of the rotation axis of the photoconductor is known (for example, see JP-A-2-4546)
As the line head, for example, JP-A-2-4546 describes an optical information writer in which a plurality of LED array chips with a plurality of LEDs (light-emitting elements) is arranged in one direction.
In the optical information writer, the plurality of LEDs of each of the LED array chips is arranged in the direction of the rotation axis of the photoconductor. Convex lens elements (optical systems) are provided so as to correspond to the respective LED array chips. The convex lens elements image the rays of light from the respective LEDs of each of the LED array chip.
In the line head described in JP-A-2-4546, due to the image-surface curvature of the convex lens element, the imaging capability of the convex lens element decreases as it becomes distant from the optical axis. On the surface of the photoconductor, a spot size of light from an LED that is located close to the optical axis of the convex lens element is different from a spot size of light from an LED that is located distant from the optical axis of the convex lens element. That is, the size of the latent image formed on the surface of the photoconductor becomes different from a pixel, which is formed by the light from the LED located close to the optical axis of the convex lens element, to a pixel, which is formed by the light from the LED located distant from the optical axis of the convex lens element. The images obtained by developing the latent image may have uneven concentration between the two pixels, and thus concentration unevenness occurs.
Furthermore, since the positional relationship between the image surface of the convex lens element and the light irradiation surface (the surface of the photoconductor) is offset or varied due to errors in mounting the line head onto the body of the image forming apparatus, eccentricity of the photoconductor, or the like, the spot size will vary. In this respect, the obtained images will have concentration unevenness.